1. Field of the Invention
This invention relates to a method of manufacturing metal pipe with longitudinally differentiated wall thickness.
2. Description of the Prior Art
Recently, exploitation of offshore oil fields (including gas fields) has been carried out at increasingly great depths. Fixed drilling platforms fastened to the seabed used to be the main equipment employed in oil and gas prospecting and exploitation. The need to work in deeper water has brought about the evolution of flexible-structure drilling platforms. One example is a tension-leg platform.
The tension-leg platform is a floating drilling platform that is secured to its anchoring members on the sea floor by means of the so-called tension legs comprising a number of steel tubular members screwed together. Usually, each tubular member is approximately 12 m long, having an external thread (a pin section) and an internal thread (a box section) cut at each end thereof. The pin and box sections are also generically called connector sections. To ensure adequate strength, the threaded connector sections at both ends are heavier in wall thickness than elsewhere. While in service, drilling platforms are subjected to everchanging forces exerted by winds, waves, currents and tides. So, the tubular members are required to have high enough fatigue strength to endure the stresses induced by such forces under seawater.
In manufacturing the tubular tension legs, the conventional practice has been to form the connector sections or the pin and box sections by forging. With the pipe having longitudinally differentiated wall thickness or outside diameter, however, such forging is not only time-consuming but also uneconomical because of the need to finish with machining.